It is frequently necessary to position objects with high precision and accuracy, and with a high reliability so that the objects can be placed in the same location repeatedly and without variation. Although there are many applications for such high-precision positioners, one in particular is the positioning of an aperture for defining an x-ray beam in, for example, a computed tomography (CT) scanner. More specifically, it is critical for diagnostic accuracy and reliability that the x-ray beam of a CT scanner be positioned precisely and reproducibly on a receiving surface of an x-ray detector. However, the position of the x-ray source may change over time due to thermal and gravitational forces on it. Accordingly, the position of the beam should be monitored and the aperture moved to compensate for changes in the position of the x-ray source so that the x-ray beam remains properly positioned. Other applications for such positioners include, for example, coordinate measuring machines, high-precision laser-cutting and laser-printing machines, and other applications in which high-precision placement of relatively modest loads is required.
A known technique for controlling the position of an x-ray beam aperture is to use a rotatable leadscrew coupled to a rotating shaft of a motor. A nut mates with the leadscrew and is attached to a slide (which includes a structure configured so as to define the aperture) that moves on a rail rigidly connected to the motor. When the leadscrew is rotated within the nut in either a clockwise or counterclockwise direction, the slide moves along the leadscrew by a corresponding amount. Such a mechanism may be used, for example, in the focal spot movement compensation system disclosed in U.S. Pat. No. 5,550,886 to Dobbs et al. and assigned to the assignee of the present invention.
There are several disadvantages to this type of mechanism. First, the movement of a nut on a leadscrew is likely to result in backlash caused by the presence of small but necessary clearances between the threads of the nut and the leadscrew. This makes it difficult to smoothly and rapidly adjust the position of the nut in both directions. Another problem is that the precise repositioning of the slide at any one position is usually not reproducible. Many potential contact points exist between the threads of the leadscrew and of the nut. As the nut moves around and along the leadscrew, the points of contact between the nut and the leadscrew are constantly changing. The repositioning of the nut at any one position on the leadscrew, and thus of the slide on the rail, is usually not reproducible. Further, additional torque is required to overcome the binding that results from the nut traveling on the leadscrew. As a result, the driver for the leadscrew must be sufficiently oversized and/or more powerful than would otherwise be necessary, and this in turn causes other components to be oversized or more powerful than would otherwise be necessary.
In addition, unless the leadscrew and the nut are always in perfect alignment, the nut will tend to bind on the leadscrew as the nut approaches the fixed end of the leadscrew. In general, the rail on which the slide moves is not exactly parallel to the leadscrew, and the leadscrew itself is generally not exactly parallel to the shaft of the motor which drives it. Therefore, the distance between the center of the leadscrew and the center of the nut is not constant as the nut moves along the leadscrew, and this can result in binding of the nut on the leadscrew.
It would thus be advantageous to provide a positioner assembly which overcomes the deficiencies of prior art leadscrew and nut mechanisms and which can provide high-precision movement of modest loads with reproducible positioning of the loads.